1. Field of the Invention
The present invention relates to coated particles and to methods of making and using them. These coated particles have application in the targeting and release of one or more materials into selected environments, the absorption of one or more materials from selected environments and the adsorption of one or more materials from selected environments.
2. Related Art
Two particle technologies—polymer-coated particles and liposomes—are of general interest.
Polymer-coated particles have been very important in the development of useful microparticles and of controlled-release vehicles generally. In certain circumstances polymers have coating and spreading properties that provide for good encapsulation of various matrices, and they are available in a range of chemistries and molecular weights. Certain polymeric coatings are of such utility and low toxicity that approval has been obtained for their use even in injectable products within the pharmaceutical industry, most notably polylactic-glycolic acid copolymers, and the usefulness of polymeric coatings in oral products is well-established, as in the cases of Eudragits, gelatin, and a number of natural gums. In many settings in fact, microparticle coatings are tacitly assumed to be polymers.
However, polymer-coated particles exhibit several limitations, as the flattened and diffuse response of their polymer coatings to chemical and physical triggers indicates. This is due to two factors. First, the high molecular weight of polymers reduces their diffusion coefficients and their kinetics of solubilization. Second, the neighboring group effect broadens the curves representing the chemical responses to triggers such as, inter alia, pH, salinity, oxidation and reduction, ionization, etc. (The neighboring group effect indicates that chemical changes in one monomeric unit of a polymer significantly alter the parameters governing chemical transitions in each of the neighboring monomeric units.) Further, most polymers are collections of chemical species of broadened molecular weight distribution. In addition, for a given application of the polymer coated particle only a limited number of suitable polymers are frequently available. This is due to a number of factors: regulatory issues: the coating processes often entail harsh chemical and/or physical conditions, such as solvents free radicals, elevated temperatures, dessication or drying, and/or macroscopic shearing forces needed to form the particles, the limited mechanical and thermal stabilities of the polymeric coatings in industrial applications; and adverse environmental impacts in large scale applications of polymer-coated particles, such as in agricultural use.
Liposomes also exhibit a number of limitations. Among these are their physical and chemical instabilities. The release of a material disposed within the liposome is usually dependent on the destabilization of the structure of the liposome. In particular, the absence of porosity precludes the pore-controlled release of such materials. The dual requirements of 1) physical stability of the liposome until release is desired on the one hand and 2) release of materials by bilayer destabilization when release is desired on the other, are problematic. (The term liposomes is frequently interchanged with the term vesicles and is usually reserved for vesicles of glycerophospholipids or other natural lipids. Vesicles are self-supported closed bilayer assemblies of several thousand lipid molecules (amphiphiles) that enclose an aqueous interior volume. The lipid bilayer is a two-dimensional fluid composed of lipids with their hydrophilic head groups exposed to the aqueous solution and their hydrophobic tails aggregated to exclude water. The bilayer structure is highly ordered yet dynamic because of the rapid lateral motion of the lipids within the plane of each half of the bilayer.) See O'Brien. D. F. and Rarnaswami, V. (1989) in Mark-Bikales-Overbergzer-Menges Encyclopedia of Polymer Science and Engineering. Vol. 17, Ed. John Wiley & Inc., p. 108.